Horse spleen ferritin is a naturally occurring iron storage protein, consisting of a
protein shell encapsulating a hydrous ferric oxide core about 8 nm in diameter. It is
known from prior work that the protein can be adsorbed onto the surface of tin-doped
indium oxide (ITO) electrodes, where it undergoes voltammetric reduction at about
–0.6 V vs Ag/AgCl. This is accompanied by dissolution of Fe(II) through channels in
the protein shell. In the present work, it is demonstrated that a pre-wave at about –0.4
V vs Ag/AgCl is due to the reduction of FePO4 also present inside the protein shell.
In order to prove that the pre-wave was due to the reduction of FePO4, it was first
necessary to prepare 8 nm diameter hydrous ferric oxide nanoparticles without protein
shells, adsorb them onto ITO electrodes, and then study their electrochemistry.
Having achieved that, it was then necessary to establish that their behaviour was
analogous to that of ferritin. This was achieved in several ways, but principally by
noting that the same electrochemical reduction reactions occurred at negative
potentials, accompanied by the dissolution of Fe(II). Finally, by switching to aqueous
phosphate buffer, the pre-wave could be unambiguously identified as the reduction of
FePO4 present as a thin layer on the hydrous ferric oxide nanoparticle surfaces.
Although the bare and protein-coated hydrous ferric oxide nanoparticles were found
to behave identically toward electrochemical reduction, they nevertheless reacted very
differently towards H2O2. The bare nanoparticles acted as potent electrocatalysts for
both the oxidation and the reduction of H2O2, whereas the horse spleen ferritin had a
much lesser effect. It seems likely therefore that the protein shell in ferritin blocks the
formation of key intermediates in hydrogen peroxide decomposition.